The BBC's Kate McGeown reports from Manila.
Alfredo Moser´s invention is lighting up the world. In 2002, the Brazilian mechanic had a light-bulb moment and came up with a way of illuminating his house during the day without electricity -using nothing more than plastic bottles filled with water and a tiny bit of bleach.
So how does it work? Simple refraction of sunlight, explains Moser, as he fills an empty two-litre plastic bottle.
"Add two capfuls of bleach to protect the water so it doesn´t turn green (with algae), The cleaner the bottle, the better", he adds.
Wrapping his face in a cloth he makes a hole in a roof tile with a drill. Then, from the bottom upwards, he pushes the bottle into the newly-made hole.
"You fix the bottle in with polyester resin. Even when it rains, the roof never leaks -not one drop."
This simple initiative in the Philippines is bringing a bit of brightness into the lives of the country´s poorest people.
In short, the project is called "Litre of Light" and the technology is just a plastic bottle filled with water.
It´s an environmentally-friendly alternative to an electric light bulb and it´s virtually free.